Gravitational redshift

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Graphic representing the gravitational redshift of a neutron star (not exact)
Graphic representing the gravitational redshift of a neutron star (not exact)

In physics, light or other forms of electromagnetic radiation of a certain wavelength originating from a source placed in a region of stronger gravitational field (and which could be said to have climbed "uphill" out of a gravity well) will be found to be of longer wavelength when received by an observer in a region of weaker gravitational field. If applied to optical wave-lengths this manifests itself as a change in the colour of the light as the wavelength is shifted toward the red (making it less energetic, longer in wavelength, and lower in frequency) part of the spectrum. This effect is called gravitational redshift and other spectral lines found in the light will also be shifted towards the longer wavelength, or "red," end of the spectrum. This shift can be observed along the entire electromagnetic spectrum.

Light that has passed "downhill" into a region of stronger gravity shows a corresponding increase in energy, and is said to be gravitationally blueshifted.

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[edit] Definition

Redshift is often denoted with the variable z\,.

z=\frac{\lambda_o-\lambda_e}{\lambda_e}

Where:

\lambda_o\, is the wavelength of the electromagnetic radiation (photon) as measured by the observer. \lambda_e\, is the wavelength of the electromagnetic radiation (photon) when measured at the source of emission.

Gravitational redshift, the displacement of light towards the red, can (for the case of a star) be predicted using the formula provided in the theory of General Relativity (Albert Einstein: Relativity - Appendix - Appendix III - The Experimental Confirmation of the General Theory of Relativity):

z_{\mathit{approx}}=\frac{GM}{c^2r}

where:

z_{\mathit{approx}}\, is the displacement of spectral lines due to gravity as viewed by a far away observer in free space. G\, is Newton's gravitational constant (the variable used by Einstein himself). M\, is the mass of the body which the light is escaping. c\, is the speed of light. r\, is the radius of star emitting the light.

GM/r is the gravitational potential at distance r, so the redshift is seen to be directly proportional to the gravitational potential. Using the energy-momentum equation relating energy and wavelength of a photon, the gravitational redshift is equivalent to a loss of energy of the photon.

[edit] History

The gravitational weakening of light from high-gravity stars was predicted by John Michell in 1783, using Isaac Newton's concept of light corpuscles (see: emission theory). The effect of gravity on light was then explored by Laplace and Johann Georg von Soldner (1801), who predicted that some stars would have a gravity so strong that light would not be able to escape. All of this early work assumed that light could slow down and fall, which was inconsistent with the modern understanding of light waves.

Once it became accepted that light is an electromagnetic wave, it was clear that the frequency of light should not change from place to place, since waves from a source with a fixed frequency keep the same frequency everywhere. The only way around this conclusion would be if time itself was altered--- if clocks at different points had different rates.

This was precisely Einstein's conclusion in 1911. He considered an accelerating box, and noted that according to the special theory of relativity, the clock rate at the bottom of the box was slower than the clock rate at the top. Nowadays, this can be easily shown in accelerated coordinates. The metric tensor in units where the speed of light is one is:

ds^2 = - r^2 dt^2 + dr^2 \,

and for an observer at a constant value of r, the rate at which a clock ticks, R(r), is the square root of the time coefficient, R(r)=r. The acceleration at position r is equal to the curvature of the hyperbola at fixed r, and like the curvature of the nested circles in polar coordinates, it is equal to 1/r.

So at a fixed value of g, the fractional rate of change of the clock-rate, the percentage change in the ticking at the top of an accelerating box vs at the bottom, is:

{R(r+dr) - R(r) \over R} = {dr\over r} = g dr \,

The rate is faster at larger values of R, away from the apparent direction of acceleration. The rate is zero at r=0, which is the location of the acceleration horizon. Locally, the rate changes in the same way as the gravitational acceleration.

Using the principle of equivalence, Einstein concluded that the same thing holds in a gravitational field, that the rate of clocks R at different heights was altered according to the gravitational field g. When g is slowly varying, it gives the fractional rate of change of the ticking rate. If the ticking rate is everywhere almost this same, the fractional rate of change is the same as the absolute rate of change, so that:

{dR \over dx} = g = - {dV\over dx} \,

Since the rate of clocks and the gravitational potential have the same derivative, they are the same up to a constant. The constant is chosen to make the clock rate at infinity equal to 1. Since the gravitational potential is zero at infinity:

R(x)= 1 - {V(x) \over c^2} \,

where the speed of light has been restored to make the gravitational potential dimensionless.

The coefficient of the dt2 in the metric tensor is the square of the clock rate, which for small values of the potential is given by keeping only the linear term:

R^2 = 1 - 2V \,

and the full metric tensor is:

ds^2 = - ( 1 - {2V(r)\over c^2} )c^2 dt^2 + dx^2 + dy^2 + dz^2

where again the c's have been restored. This expression is correct in the full theory of general relativity, to lowest order in the gravitational field, and ignoring the variation of the space-space and space-time components of the metric tensor, which only affect fast moving objects.

The changing rates of clocks allowed Einstein to conclude that light waves change frequency as they move, and the frequency/energy relationship for photons allowed him to see that this was best interpreted as the effect of the gravitational field on the mass-energy of the photon.

Einstein was accused by Philipp Lenard of plagiarism for not citing Soldner's earlier work - a baseless accusation motivated by Lenard's vocal anti-semitism. Einstein's result was based on gravitational time dilation, an idea which had no antecedant in any earlier work, and without which a correct theory of gravitational light deflection is impossible.

[edit] Important things to stress

  • The receiving end of the light transmission must be located at a higher gravitational potential in order for gravitational redshift to be observed. In other words, the observer must be standing "uphill" from the source. If the observer is at a lower gravitational potential than the source, a gravitational blueshift can be observed instead.
  • Tests done by many universities continue to support the existence of gravitational redshift.[citation needed]

[edit] Initial verification

A number of experimenters initially claimed to have identified the effect using astronomical measurements, and the effect was eventually considered to have been finally identified in the spectral lines of the star Sirius B by W.S. Adams in 1925. However, measurements of the effect before the 1960s have been critiqued by (e.g., by C.M. Will), and the effect is now considered to have been definitively verified by the experiments of Pound, Rebka and Snider between 1959 and 1965.

The Pound-Rebka experiment of 1959 measured the gravitational redshift in spectral lines using a terrestrial 57Fe gamma source. This was documented by scientists of the Lyman Laboratory of Physics at Harvard University. A commonly-cited experimental verification is the Pound-Snider experiment of 1965.

More information can be seen at Tests of general relativity.

[edit] Application

Gravitational redshift is studied in many areas of astrophysical research.

[edit] Exact Solutions

A table of exact solutions of the Einstein field equations consists of the following:

Non-rotating Rotating
Uncharged Schwarzschild Kerr
Charged Reissner-Nordström Kerr-Newman

The more often used exact equation for gravitational redshift applies to the case outside of a non-rotating, uncharged mass which is spherically symmetric. The equation is:

z=\frac{1}{\sqrt{1-\left(\frac{2GM}{rc^2}\right)}}-1, where

[edit] Gravitational Redshift vs. Gravitational Time Dilation

When using special relativity's relativistic Doppler relationships to calculate the change in energy and frequency (assuming no complicating route-dependent effects such as those caused by the frame-dragging of rotating black holes), then the Gravitational redshift and blueshift frequency ratios are the inverse of each other, suggesting that the "seen" frequency-change corresponds to the actual difference in underlying clockrate. Route-dependence due to frame-dragging may come into play, which would invalidate this idea and complicate the process of determining globally-agreed differences in underlying clock rate.

While gravitational redshift refers to what is seen, gravitational time dilation refers to what is deduced to be "really" happening once observational effects are taken into account.

[edit] Primary sources

  • John Michell "On the means of discovering the distance, magnitude etc. of the fixed stars" Philosophical Transactions of the Royal Society (1784) 35-57, & Tab III
  • R.V. Pound and G.A. Rebka, Jr. "Gravitational Red-Shift in Nuclear Resonance" Phys. Rev. Lett. 3 439-441 (1959)
  • R.V. Pound and J.L. Snider "Effect of gravity on gamma radiation" Phys. Rev. 140 B 788-803 (1965)
  • R.V. Pound, "Weighing Photons" Classical and Quantum Gravity 17 2303-2311 (2000)

[edit] See also